In certain geographical areas hurricanes are a constant threat during certain seasons of the year. One source of great damage from hurricanes is that wind pressures against window panes frequently cause the panes to break, allowing wind and water to enter. In preparation for a hurricane, when advance warning has been given, many building owners laboriously apply tape in criss-cross patterns to window panes. The application of tape serves only to reduce the danger from flying glass when a window pane breaks but adds little to the ability of the window pane to resist wind pressure.
The properties of glass are well known. For a given type of glass there is a known stress at which fracture will occur. The stress in the glass is directly proportional to the magnitude and direction of the load applied and the strain thus produced. The onset of fracture usually results from the propagation of microminiature cracks, found mainly at the edges, where unsupported glass has the highest stress concentrations. The distribution of stress is also highly dependent upon the geometry of the glass which is characterized by what is called the alpha characteristic. Different shaped windows with the same area will withstand different loads because the stress distribution, and particularly the stress level at the edges, is determined by the alpha characteristic.
In a high wind situation there are three different types of loading on the windows of a building. The front of the building facing into the wind is subjected to inward pressure due to the "head-on force", the side windows are subjected to outward pressure due to the Bernoulli effect and the back windows are subjected to complicated forces due to turbulence. In hurricane conditions the magnitude of the force exerted on a large window reaches the level of thousands of pounds.
The maximum stress in a supported glass window of minimum dimension b is approximately proportional to (b - BD).sup.2 where B is a constant and D is the diameter of the supporting plate. For a given type of glass the breaking stress is tabulated and from the dimensions of the window the constants of proportionality may be calculated. Thus, given a maximum force anticipated, the minimum value of D may be obtained.
By incorporating a suction device into the plate outward forces can also be resisted. The maximum resisting force that the suction device can exert is directly proportional to the diameter of the evacuated region d, squared. Thus, from a maximum anticipated force outwards, which is calculated quite differently from the inward force, a critical value of d can be determined. For both resistive inward and outward forces it is necessary to distribute the pressure over a large enough area to reduce stress concentrations.
For unsupported windows of a given type of glass and dimensions there is a critical deflection of the center of the window which gives rise to the breaking stress being reached. In principle the center of the window should not be allowed to deflect at all, thereby effectively reducing the dimensions of the glass. In practice, for reasons of economy and weight, a maximum deflection equal to a small fraction of the breaking deflection is allowed. Given this maximum allowable deflection at the center and a given material for the brace, the dimensions and cross section may be determined from well known formulae.
The present invention provides a means for reinforcing a window pane against wind pressure and has particular application in those geographical areas wherein periodic high wind pressures can be anticipated.
The invention provides a simple, inexpensive and easily applied apparatus for reinforcing a window pane against breakage from wind pressure. The apparatus has the advantage that when not required, such as during periods of the year when high wind velocities are not anticipated, the apparatus can easily be removed so as not to interfere with normal unobstructed view and light passage through the window.
The device is designed to increase the load either inward or outward that a window can withstand by:
a. transferring a large fraction of the load to the structural members of the building and
b. distributing the remaining load evenly throughout the glass rather than permitting areas of large stress concentration.
It is therefore an object of this invention to provide an apparatus for reinforcing a window pane against breakage from high wind pressure.
Another object of this invention is to provide an apparatus which is easily affixed to and removed from a window frame for reinforcing the window pane within the frame against breakage from high wind pressures.
Another object of the invention is to provide an apparatus for reinforcing a window pane against breakage as a result of high wind pressures including means wherein the reinforcing pressure applied against the window pane is adjustable.
Another object of the invention is to provide an apparatus for reinforcing a window pane against breakage as a result of high positive or negative air pressures, that is, air pressure tending to blow the window pane into or out of a building.
These objects, as well as others, will be fulfilled in the description and claims which follow, taken in conjunction with the attached drawings.